The invention relates to a passive electric structural element with an adjustable capacitance value, commonly called a trimming capacitor.
Adjustable capacitors are usually composed of a stator with a sector-shaped ring electrode covering the surface at an angle of about 180.degree.. The connection of the electrode to the armature produced by a soldering connection lies in a depression.
The rotor is composed of high-grade capacitor ceramic and also has a sector-formed ring electrode which is in contact with the rotor shaft through a soldering connection. The stator and rotor are adjusted to each other through the rotor shaft. It is possible to turn the capacitor layers towards each other at the same time by turning the two structural elements in such a way that the capacitance value of the structural element can be set between an upper and a lower limiting value.
Silver is preferred as a manufacturing material for the electrodes, which is affixed through spraying, screen printing or ram pressing, followed by annealing. The electrodes and especially the stator electrodes as in normal ceramic capacitors, must have high electrical conductivity in order to fulfill the required electrical parameters, especially those regarding small angle losses.
Adjustable capacitors must allow a certain number of capacitance value settings so that the stator electrode must also fulfill certain requirements regarding abrasion resistance. If silver is employed as the electrode material, this requirement is fulfilled by relatively thick silver layers in the range of 5 to 10 .mu.m. Since the stator only fulfills the function of a carrier for the stator electrode, ceramic which has larger surface roughness values opposite the rotor is used for this purpose. Nevertheless, a sufficiently even surface for the stator electrode is obtained through the thickness and the annealing process of the silver layer in conjunction with smoothing of the silver coating.
Silver in the indicated thickness as the electrode material, also completely fulfills the requirements concerning solderability for affixing the connection armatures.
In contrast to capacitors with fixed values, which are generally protected by coverings from the damaging effects of the atmosphere, especially heat and humidity, trimming capacitors should not be covered in order to permit adjustment. The electrodes thus are fully exposed to the damaging effects of the atmosphere. Silver as electrode material has sufficient characteristics in this regard. One definite disadvantage in using silver as an electrode material is its high price, which initially affects the price of the structural element in a distinctive manner. For this reason, other electrode materials have already been proposed for capacitors with fixed values, such as, for instance, nickel or copper, which, however, do not fulfill even one of the conditions required for trimming capacitors.
Thus, for instance, copper is not sufficiently resistant to wear and is also not corrosion-proof. Nickel requires the application of corrosive soldering fluxes when soldering the armatures, which leads to substantial deterioration of the loss angle.
The purpose of the invention is to create a trimming capacitor which can be produced economically without employing noble metals.